♻️ Rename primitive to primitive-union

`getMatcher` / `getMatcherW` become `getPrimitiveUnionMatcher` /
`getPrimitiveUnionMatcherW`; `src/primitive.ts` and its test move to
`src/primitive-union.*`. The `Union` suffix mirrors `getTaggedUnionMatcher`.

src/primitive-union.ts

@@ -0,0 +1,131 @@
+import type { Exact } from "type-fest";
+
+import type {
+    HandlerMap,
+    PatternReturns,
+    RedundantFallback,
+    UnaryFn,
+} from "./matcher-shared.ts";
+
+// The primitive universe a matcher can discriminate. `boolean` is admitted as
+// the pair `true | false`; see README § Caveats for the unsupported members.
+type Matchable = string | number | boolean | null | undefined;
+
+// `boolean`, `null` and `undefined` cannot be property keys, so a mapped type
+// over the universe keys each non-key member by its stringification. `Param`
+// inverts that projection, so a handler callback still receives the *real*
+// member (`true`, not `"true"`) — see README § Caveats for the limits.
+type PatternKey<T> = T extends boolean
+    ? T extends true
+        ? "true"
+        : "false"
+    : T extends null
+      ? "null"
+      : T extends undefined
+        ? "undefined"
+        : T;
+type PatternParam<K> = K extends "true"
+    ? true
+    : K extends "false"
+      ? false
+      : K extends "null"
+        ? null
+        : K extends "undefined"
+          ? undefined
+          : K;
+
+type Handlers<T extends Matchable, R> = {
+    [K in PatternKey<T>]: UnaryFn<PatternParam<K>, R>;
+};
+
+// The fallback is a *second argument*, not a property of the handler map,
+// because its parameter is the remainder `Exclude<T, keyof Handled>` and TypeScript
+// fixes a property's contextual type before it infers its sibling keys. A later
+// argument, by contrast, is contextually typed from inference on an earlier
+// one, so the split is what makes the remainder expressible at all.
+// See development/library.md.
+type Fallback<T extends Matchable, Handled, R> = UnaryFn<
+    Exclude<T, PatternParam<keyof Handled>>,
+    R
+>;
+
+// A fallback is redundant once the handler map covers `T`. The guard is folded
+// into `Handled`'s own (self-referential) constraint so it is checked *after*
+// inference; a conditional in the fallback's parameter type is evaluated while
+// `Handled` is still its constraint and would reject context-sensitive partial
+// maps. That placement also fixes where the diagnostic lands: the constraint
+// failure is reported on the argument that inferred `Handled` (the handler
+// map), so the required property is spelled as the message instead of relying
+// on its position. See development/library.md.
+type MustBePartial<T extends Matchable, Handled> =
+    PatternKey<T> extends keyof Handled ? RedundantFallback : unknown;
+
+// TypeScript does not apply the excess-property check to a generic constraint,
+// so `Exact` restores it for the generic forms: a handler map can otherwise
+// carry keys outside `T`.
+
+// oxlint-disable typescript/unified-signatures
+// Strict returns: one common `R`. Overload order is load-bearing:
+//   #1 Handlers (first) -> the exhaustive form and the autocomplete popup
+//   #2 Fallback (last)  -> accepts a partial handler map plus a fallback
+interface MatcherStrict<T extends Matchable> {
+    <R>(handlers: Handlers<T, R>): UnaryFn<T, R>;
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, R>>, Handled> &
+            MustBePartial<T, Handled>,
+    >(
+        handlers: Handled & Partial<Handlers<T, R>>,
+        fallback: Fallback<T, Handled, R>,
+    ): UnaryFn<T, R>;
+}
+// oxlint-enable typescript/unified-signatures
+
+// Widened returns: the union of every handler's return type. `P` is inferred
+// from the whole handler map, whose closed constraint supplies the
+// contextual/autocomplete type.
+// oxlint-disable typescript/unified-signatures
+interface MatcherWidening<T extends Matchable> {
+    <P extends Exact<Handlers<T, unknown>, P>>(
+        handlers: P,
+    ): UnaryFn<T, PatternReturns<P>>;
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, unknown>>, Handled> &
+            MustBePartial<T, Handled>,
+    >(
+        handlers: Handled,
+        fallback: Fallback<T, Handled, R>,
+    ): UnaryFn<T, PatternReturns<Handled> | R>;
+}
+// oxlint-enable typescript/unified-signatures
+
+const dispatch =
+    (handlers: HandlerMap, fallback?: UnaryFn<never, unknown>) =>
+    (shape: Matchable): unknown =>
+        // `handlers[true]` already coerces to the `"true"` property at runtime,
+        // identical to `handlers[String(shape)]`, so indexing with `shape`
+        // directly is sound: `shape` is a facade-checked universe member and
+        // `PatternKey` only ever produces valid property keys. The assertion is
+        // needed solely because TypeScript forbids indexing with
+        // `boolean`/`null`/`undefined` (TS2538); it buys the number fast path.
+        (
+            handlers[
+                // oxlint-disable-next-line typescript/no-unsafe-type-assertion
+                shape as string | number
+            ] ??
+            fallback ??
+            (() => {
+                throw new Error(`Unhandled shape: ${String(shape)}`);
+            })
+        )(
+            // oxlint-disable-next-line typescript/no-unsafe-type-assertion
+            shape as never,
+        );
+
+export const getPrimitiveUnionMatcher = <
+    T extends Matchable,
+>(): MatcherStrict<T> => dispatch;
+export const getPrimitiveUnionMatcherW = <
+    T extends Matchable,
+>(): MatcherWidening<T> => dispatch;